The invention relates to a method for analyzing a primary neutron beam of a neutron source, a neutron source having a beam monitor, and a beam monitor for neutron beams.
It is known to use fission chambers as beam monitors for the purpose of analyzing the primary neutron beam of a neutron source. Such fission chambers have on their inner walls a thin layer of 235U-containing material which serves as neutron absorber. Neutrons captured by 235U cause thermal fission in which the desired fission products produce electrons and ions in a gas located in the fission chamber. Because of an applied electric voltage, these mobile charge carriers effect an ionization current pulse with the aid of which the neutron producing the fission is counted.
A disadvantage of the fission chambers is that in addition to the desired fission products three fast MeV neutrons on average and MeV gamma radiation are produced during thermal fission. This high-energy neutron and/or gamma radiation is undesired as a rule in the case of neutron sources.
Also disadvantageous is that the flux of slow neutrons through the fission chamber can be substantially attenuated, since the neutron absorption is inversely proportional to the speed of the neutrons.
It is the object of the present invention to make available a method, a neutron source and a beam monitor of the type mentioned at the beginning which do not have the abovementioned disadvantages.
In the case of a method of the type mentioned at the beginning, this object is achieved by virtue of the fact that a portion of the neutrons are scattered out of the primary neutron beam by means of a scattering unit which scatters neutrons at least predominantly incoherently, and a portion of the neutrons which can be defined via the scattering behaviour of the scattering unit and the geometrical conditions are fed to a detector device arranged outside the primary neutron beam. This prevents the primary neutron beam from being disturbed by a detecting operation required for analyzing this neutron beam. The primary neutron beam can be screened effectively from radiation possibly produced in the detector device. The incoherent scattering of the neutrons from the primary neutron beam can be selected such that the primary neutron beam is attenuated only insubstantially. Thus, for example, the fraction of the scattered neutrons in the total number of the neutrons in the primary neutron beam can be fixed at values of between 10xe2x88x922 and 10xe2x88x926.
The method according to the invention can also be designed such that the neutrons in the detector device are guided through a collimator unit with the result that only neutrons essentially parallel in their direction of movement are detected in a detecting unit arranged downstream of the collimator unit in the direction of movement of the neutrons, and that the impingement of the neutrons is determined in a spatially resolved fashion in the detecting unit.
If only such neutrons as have essentially parallel directions of movement are detected in the detecting unit, the spatially resolved detection of the neutrons in the detecting unit can be used to gain information on the spatial distribution of the neutrons in the scattering unit and thus in the primary neutron beam. An essentially parallel direction of movement means that the direction of movement of the detected neutrons deviates from the collimator axis at most by an acceptance angle prescribed by the design of the collimator. The acceptance angle can be determined, for example, depending on the required spatial resolution at values of between 10xe2x88x921 and 10xe2x88x922 radians.
The method according to the invention can be designed such that in the case of a pulsed neutron source the instants of the detection of the neutrons in the detector device are determined, and time of flight spectra are prepared with reference to the instant of the production of the neutrons. A time-resolved analysis of the primary neutron beam is thereby also rendered possible.
With regard to a neutron source having a beam monitor for investigating a primary neutron beam produced in the neutron source, the beam monitor comprising a detector device for detecting neutrons, the abovenamed object is achieved by virtue of the fact that the beam monitor has a scattering unit arranged in the beam path of the primary neutron beam and scattering neutrons at least predominantly incoherently, and in that the detector device is arranged outside the beam path of the primary neutron beam and provided for the purpose of analyzing a portion of the neutrons scattered by the scattering unit.
It can be advantageous to construct the neutron source according to the invention such that the detector device comprises
a) a collimator unit which allows the passage only of neutrons with a direction of movement essentially parallel to the collimator axis,
b) a detecting unit arranged downstream of the collimator unit seen in the direction of movement of the scattered neutrons and suitable for detecting impinging neutrons, and
c) means for determining the spatial distribution of the neutrons detected by the detecting unit.
The neutron source according to the invention can also be constructed such that the detecting unit is a scintillator, and in that for the purpose of detecting the spatial distribution of the detected neutrons at least one spatially resolving photomultiplier is provided downstream of the scintillator seen in the direction of movement of the neutrons impinging on the scintillator, or a plurality of photomultipliers are arranged in an array parallel to the side of, the scintillator facing the scattering unit. The scintillator can be selected to be so thick that all the neutrons within the wavelength band of 0.01 to 1 nm are absorbed up to at least 70%. The absorbed neutrons produce a light pulse which can be evaluated by the photomultiplier or photomultipliers. In the case of an arrangement of non-spatially resolving photomultipliers in the form of an array, each individual detection event is assigned to the photomultiplier with the aid of which the strongest pulse is determined, thus securing the spatial resolution which becomes better the more photomultipliers are used for detecting. Alternatively, it is also possible to make use of a spatially resolving photomultiplier. A spatially resolving photomultiplier is already enough to permit a spatial resolution of the neutron detection on the region of the scintillator it is monitoring.
The neutron source according to the invention can also be constructed such that the scattering unit is a two-dimensional foil, the collimator axis is perpendicular to the scattering foil, and the surface of the detecting unit facing the scattering foil is parallel to the scattering foil. Since the surface of the detecting unit on which the neutrons impinge is thus parallel to the scattering foil, and the detected neutrons have essentially parallel directions of movement, the path length of these neutrons between the scattering foil and detecting unit is essentially the same, that is to say independent of the location of the scattering and/or of the location of the detection, for which reason the spatial sensitivity of the beam monitor is not disturbed by different path lengths.
However, the neutron source according to the invention can also be constructed such that the collimator axis is perpendicular to the direction of movement of the neutrons in the primary beam path, and the surface of the detecting unit facing the scattering foil is perpendicular to the collimator axis. In this arrangement, it is not possible to achieve parallelism between the scattering foil and the surface of the detecting unit facing the scattering foil, that is to say the distance between the detecting unit and scattering foil varies. However, since this variation is time-stable, it can be corrected computationally. Since in the case of this arrangement the direction of movement of the neutrons to be detected in the detecting unit is perpendicular to the course of the primary beam path, the length of the beam monitor seen in the direction of the primary neutron beam can be minimized. It is also possible as a further alternative in the case of a collimator axis aligned perpendicular to the course of the primary neutron beam to arrange the detecting unit and the scattering foil parallel to one another. In this case, the collimator axis is not aligned at right angles to the two-dimensional extent of the collimator or to the impingement surface of the detecting unit.
Finally, the neutron source according to the invention can be constructed such that the neutron source is a pulsed spallation neutron source. In the case of pulsed spallation neutron sources, in particular, a spectral and spatial analysis of the primary neutron beam is desired.
In the case of a beam monitor for neutron beams which comprises a detector device suitable for detecting neutrons, the abovenamed object is achieved by a scattering unit which scatters neutrons at least predominantly incoherently. Incoherent scattering can, for example, be produced by scattering foils made from vanadium, polyethylene or from a titanium isotope mixture which exhibit only slight Bragg reflection or none.
Finally, the beam monitor according to the invention can also be constructed such that the detector device comprises
a) a collimator unit which allows the passage only of neutrons with a direction of movement essentially parallel to the collimator axis,
b) a detecting unit arranged downstream of the collimator unit seen in the direction of movement of the penetrating neutrons and suitable for detecting impinging neutrons, and
c) means for determining the spatial distribution of the neutrons detected by the detecting unit.